A simplified diagrammatic representation of a disk drive, generally designated as 10, is illustrated in FIG. 1. The disk drive 10 includes a disk stack 12 (illustrated as a single disk in FIG. 1) that is rotated by a spindle motor 14. The spindle motor 14 is mounted to a base plate 16. An actuator arm assembly 18 is also mounted to the base plate 16.
The actuator arm assembly 18 includes a transducer 20 (or head) mounted to a flexure arm 22 which is attached to an actuator arm 24 that can rotate about a pivot bearing assembly 26. The actuator arm assembly 18 also includes a voice coil motor 28 which moves the transducer 20 relative to the disk 12. The spin motor 14, and actuator arm assembly 18 are coupled to a number of electronic circuits 30 mounted to a printed circuit board 32. The electronic circuits 30 typically include a digital signal processor (DSP), a microprocessor-based controller and a random access memory (RAM) device.
Referring now to the illustration of FIG. 2, the disk stack 12 typically includes a plurality of data storage disks 34, each of which may have a pair of disk surfaces 36, 36. The disks 34 are mounted on a cylindrical shaft and are designed to rotate about axis 38. The spindle motor 14 as mentioned above, rotates the disk stack 12.
Referring now to the illustration of FIGS. 1 and 3, the actuator arm assembly 18 includes a plurality of the transducers 20, each of which correspond to one of the disk surfaces 36. Each transducer 20 is mounted to a corresponding flexure arm 22 which is attached to a corresponding portion of the actuator arm 24 that can rotate about the pivot bearing assembly 26. The VCM 28 operates to move the actuator arm 24, and thus moves the transducers 20 relative to their respective disk surfaces 36.
Although the disk stack 12 is illustrated having a plurality of disks 34, it may instead contain a single disk 34, with the actuator arm assembly 18 having a corresponding single actuator arm 24.
FIG. 4 further illustrates one of the disks 34. Data is stored on the disk 34 within a number of concentric tracks 40 (or cylinders). Each track is divided into a plurality of radially extending spokes 42 on the disk 34. Each spoke 42 is further divided into a servo spoke 44 and a data spoke 46. The servo spokes 44 of the disk 34 are used to, among other things, accurately position the transducer 20 so that data can be properly written onto and read from the disk 34. The data spokes 46 are where non-servo related data (i.e., user data) is stored and retrieved. Such data, upon proper conditions, may be overwritten.
FIG. 5 illustrates portions of tracks 40 at radial locations n−1 to n+4, and which are drawn in a straight, rather than arcuate, fashion for ease of depiction. To accurately write data to and read data from the data spokes 46 of the disk 34, it is desirable to maintain the transducer 20 in a relatively fixed position with respect to a given data track's centerline 48 during each of the writing and reading procedures (called a track following operation).
To assist in controlling the position of the transducer 20 relative to the data track centerline 48, the servo spokes 44 can contain a servo preamble and servo burst patterns 50. The servo preamble can include a write/read (W/R) recovery field, an automatic gain control (AGC) field, a synchronization field, a spoke (sector) number field, and/or a cylinder number field. Fields of a servo spoke are illustrated in U.S. Pat. No. 6,256,160, which is incorporated herein by reference. Unlike information in the data spokes 46, the servo spokes 44 should not be overwritten or erased during normal operation of the disk drive 10.
The W/R recovery field can be used by the disk drive 10 to transition from writing data to a previous data track to reading information in the present servo spoke 44. The AGC field can be used to set a gain of the read/write channel of the disk drive 10. The synchronization field can be used to synchronize a clock so that spoke (sector) and cylinder number fields can be read, and so that the servo burst patterns 50 can be located. The spoke number field can be indicative of the circumferential position of the servo region with respect to the disk 34. The cylinder number field can be indicative of a radial location of the servo region.
The servo burst patterns 50 can include one or more groups of servo bursts, as is well-known in the art. A servo burst pattern 50 that includes first, second, third and fourth servo bursts A, B, C and D, respectively, are shown in FIG. 5. The servo bursts A, B, C, D are accurately positioned relative to each other.
During the manufacturing process of the disk drive 10, a servo-track writer (“STW”) (not shown) can is used to write servo bursts A, B, C, D onto each of the servo spokes 44 of the disk 34. In FIG. 5, the distance (d) between each pair of horizontal grid lines represents ½ of the servo track pitch. Accordingly, each of the servo bursts A, B, C, D depicted in FIG. 5 spans a distance equal to the track pitch (or one track width). Additionally, as depicted in FIG. 5, the transducer 20 has a width approximately equal to one-half of the data track width. The transducer 20 is shown to be misaligned from the track centerline 48 of track n−1 to more clearly illustrate an example of its width.
As the transducer 20 is positioned over a track 40, it reads the servo information contained in the servo spokes 44 of the track 40 as a read signal, one servo spoke 44 at a time. The servo information contained in the read signal is used to generate position error signals as a function of the misalignment between the transducer 20 and the track centerline 48. The position error signals are provided to a servo controller that performs calculations and outputs a servo compensation signal which controls the voice-coil motor 28 to position the transducer 20 relative to the track centerline 48.
When the transducer 20 is positioned exactly over the centerline 48 of track n, approximately one-quarter of the A burst will be read followed by one-quarter of the B burst, and their amplitudes will be equal in the read signal. As the transducer 20 moves off-track (i.e., off of the track centerline), the amplitude of one burst will increase while the amplitude of the other burst will decrease, depending on the direction of misalignment. Accordingly, the radial position of the transducer 20 relative to the tracks can be determined based the servo information in the read signal from the servo bursts A, B, C, D.
FIG. 6 shows the servo burst patterns of FIG. 5 in more detail. The horizontal grid markings in FIG. 6 represent half-track spacings, while the vertical grid patterns represent burst cell times. Each of the servo bursts A, B, C, D includes a plurality of pulses which have a length of one burst cell time. Reference is made, for example, to pulses 60-74 of servo burst A between the centerlines of track n−1 and track n. Adjacent pulses (for example, pulse 60 and pulse 61) have magnetic domains that are inverted (i.e., 180 degrees out of phase from one another). The pulses with cross-hatching from the lower left to the upper right (e.g., 60, 62, 64, 66, 68, 70, 72, and 74) have a first polarity, while the pulses with cross-hatching from the upper left to the lower right (e.g., 61, 63, 65, 67, 69, 71, and 73) have a second polarity that is opposite to the first polarity.